This invention relates to low expansion, transparent glass-ceramics, glass-ceramic substrates and optical waveguide elements comprising such low expansion, transparent glass-ceramics.
Known in the art of glass-ceramics are several types of low expansion, transparent glass-ceramics formed by melting and heat treating a base glass of a SiO2—Al2O3—Li2O system containing a nucleating agent.
For example, U.S. Pat. No. 3,499,773 discloses low expansion transparent glass-ceramics selectively providing a non-transparent portion which is obtained from a base glass of a SiO2—Al2O3—Li2O system containing one or more of TiO2, ZrO2 and SnO2 as a nucleating agent and optionally a small amount of MgO, CaO, SrO, BaO etc. For manufacturing the glass-ceramics disclosed in this patent, the melting temperature was 1537° C. to 1593° C. (Example 1: 2800° F. to 2900° F.). U.S. Pat. No. 4,341,543 discloses transparent glass-ceramics obtained from a base glass of a SiO2—Al2O3—Li2O system containing TiO2 and ZrO2 as nucleating agents. For manufacturing the glass-ceramics disclosed in this patent, the melting temperature was 1650° C.
In manufacturing these glass-ceramics disclosed in the patent specifications of the above mentioned patents, however, a relatively high temperature is required for melting the base glasses which results in difficulty in producing glass-ceramics having an excellent optical homogeneity in a large scale.
U.S. Pat. No. 3,681,102 describes that glass-ceramics of a SiO2—Al2O3—ZnO system containing spinel crystal as a predominant crystal phase and having an average linear thermal expansion coefficient α within a range from 25×10−7 to 40×10−7 were obtained at a melting temperature within a range from 1650° C. to 1800° C. Japanese Patent Application Laid-open Publication No. Hei 11-335139 describes that glass-ceramics of a SiO2—Al2O3—ZnO system containing spinel crystal as a predominant crystal phase and having an average linear thermal expansion coefficient α within a range from 33×10−7 to 40×10−7 were obtained at a melting temperature within a range from 1600° C. to 1625° C. These glass-ceramics, however, require a high melting temperature of 1600° C. or over and, moreover, contain gahnite (ZnAl2O4) which is known to be a very hard crystal and, therefore, have difficulty in polishing.
Japanese Patent Application Laid-open Publication Nos. Hei 10-321759 and Hei 10-321760 describe that low expansion glass-ceramics were obtained at a relatively low melting temperature. These glass-ceramics, however, are all opaque glass-ceramics and no transparent glass-ceramics were obtained.
An optical waveguide element generally consists of a lower clad, a core and an upper clad disposed on a substrate. SiO2 clads made of SiO2 are used for the lower and upper clads, a SiO2—GeO2 core made by doping GeO2 is used for the core, and a Si single crystal wafer or quartz is used for the substrate (Japanese Patent Application Laid-open Publication Nos. Hei 7-113923, Hei 11-2733 and 2000-121867).
There is a recent tendency toward seeking higher physical and chemical properties for low expansion transparent glass-ceramics. These properties include:    (1) Melting and refining of a base glass should be easy and, for this purpose, materials of the base glass should not include cord, foam or inclusion but should have a high optical homogeneity.    (2) Precipitated crystal should be fine and have excellent transparency, particularly excellent transmittance in the visible rays region.    (3) Materials should not substantially contain Na2O or K2O, since, if the materials contain Na2O or K2O, ions of such ingredient scatter during processing of the glass resulting in difficulties.    (4) Low expansion transparent glass-ceramics generally contain a relatively large amount of SiO2 for achieving required properties and hence a relatively high melting temperature of 1600° C. or over is generally required. From the standpoint of manufacturing process design and quality control, however, a lower melting temperature is preferable.
In a case where a Si single crystal wafer is used as a substrate for an arrayed waveguide grating (AWG) type planar lightwave circuit using an optical waveguide element, the surface of the substrate is exposed to a temperature exceeding 800° C. in the manufacturing process and, for this reason, deformation or distortion of the substrate takes place due to difference in thermal expansion between the substrate and layers provided on the substrate and such deformation and distortion in turn cause change in light wavelength and impairs flatness of the substrate. Even in a normal state of use, difference in a coefficient of thermal expansion between Si single crystal and SiO2 produces stress in the arrayed waveguide grating resulting change in refractive index of the arrayed waveguide grating and this necessitates insertion of a wavelength sheet in the center of the arrayed waveguide grating. There is an additional disadvantage in using the Si single crystal as the substrate that adjustment of temperature becomes necessary.
On the other hand, use of quartz as the substrate is disadvantageous, as compared with the Si single crystal, in respect of heat conductivity, mechanical properties and easiness of processing.
It is, therefore, an object of the invention to eliminate the above described disadvantages of the prior art technique and dissolve the problem that glass-ceramics which may overcome such disadvantages require a high melting temperature.
It is another object of the invention to provide low expansion transparent glass-ceramics and glass-ceramic substrates which can be obtained at a relatively low melting temperature, more specifically 1530° C. or below.
It is another object of the invention to provide an optical waveguide element, more particularly an arrayed waveguide grating (AWG) type planar lightwave circuit, employing such low expansion transparent glass-ceramics which can maintain necessary properties in a hybrid integrated circuit and can be manufactured at a low cost.